This invention relates to message store-and-forward communication systems and particularly to radio-frequency store-and-forward communication systems which operate typically over a predetermined, limited number of available communication channels. Features of the invention are found to have particular application in satellite communication systems operating in a xe2x80x9cstar topologyxe2x80x9d over a fixed number of channels.
The invention herein is described as an improvement over a well known prior art system which is known as the xe2x80x9cStandard-C communication systemxe2x80x9d, see THE STANDARD C COMMUNICATION SYSTEM, N. Teller et al. International Maritime Satellite Organisation, London, England, International Conference on Satellite Systems for Mobile Communications and Navigation, 4th, London, England, Oct. 17-19, 1988, Proceedings (A89-36576 15-32). London, Institution of Electrical Engineers, pp. 43-46, (1988). A communication system such as the Standard-C system operates in a xe2x80x9cstar topologyxe2x80x9d, hence between a xe2x80x9chubxe2x80x9d and a substantial number of xe2x80x9cmobile terminalsxe2x80x9d. The Standard-C system utilizes schemes known as Time-Division Multiple-Access (TDMA) and Frequency-Division Multiple-Access (FDMA) to accommodate a large number of intermittent users (intermittently used mobile terminals) to share a limited-bandwidth, allocated, radio frequency band.
In FDMA, the allocated band is divided into a first number of narrow sub-bands, each sub-band constituting a time-continuous channel. The first number of channels are to be shared among a second number of users, where the second number, the number of users, is typically much larger than the first number of channels. An access protocol referred to as trunking is used to accommodate the relatively larger number of users.
In TDMA, an entire allocated band forms a wideband communication channel which is allocated to different users at different times. Data packets from a given user may be interspersed with those of another user during transmission over the communication channel.
Known communication systems, including the above-identified Standard-C Communication System, use a combination of FDMA and TDMA, where time-division multiplexing is used in the frequency-division multiplexed sub-bands. In the Standard-C system, forward (from the hub to the mobile terminals) data traffic is carried in a time-division multiplexed (TDM) forward channel which is received by all mobile terminals of the system. Other, combined frequency/time-division multiplexed channels carry return data traffic from the mobile terminals to the hub.
Both the forward data traffic and return data traffic consist of two traffic components of data. A first traffic component of data necessary for xe2x80x9ccall-setupxe2x80x9d and xe2x80x9ccall-teardownxe2x80x9d includes data referred to broadly as system data and more specifically as network management data. The first traffic component of data is called xe2x80x9csignallingxe2x80x9d or signalling packets (of data). The term xe2x80x9csignallingxe2x80x9d is used herein throughout to refer to this first traffic component of data and to a specific transmission mode in communication from the mobile terminals to the hub. A second traffic component of data bears user information, such as messages or data reports. Messages are typically user information, having been composed by the user, while data reports are typically telemetry-type information packets that are transmitted periodically by the mobile terminals. The transmissions of the second traffic component of data to transfer user messages is also referred to herein as xe2x80x9cmessagingxe2x80x9d. User information is in the forward direction communicated over typically fixed links to the hub. The hub stores the user information and selectively formats it as the second traffic component of data into the frames for transmission over the TDM forward channel. Systems, such as the Standard-C system are therefore also referred to as xe2x80x9cstore-and-forwardxe2x80x9d communication systems.
The Standard-C system communicates at a fixed data rate of 600 bits per second (bps). In communication over the TDM, or forward channel, data of both components are formatted into frames and are transmitted as a sequence of consecutive frames over the TDM channel. The frame length is established at 8.64 seconds, such that during a 24-hour period an integer number of 10,000 frames are transmitted over the TDM channel. The information, message and signalling packets, are scrambled, xc2xd-rate convolutionally encoded and interleaved on a frame by frame basis. Decoding of received frames of information is also done on a frame by frame basis. Considering that the transfer of messages or xe2x80x9cmessagingxe2x80x9d also requires xe2x80x9csignallingxe2x80x9d in both the forward and the return direction, decoding delays become additive and result in typical message transport delays of several minutes, such that a command-response type of transaction cycle may take place over a time period of about five minutes.
Past applications of the Standard-C system in global communications have traditionally involved communications between a fixed shore station (the hub) and any one of a number of mobile terminals which were ship-based. In such maritime environment, message transport delays of several minutes were not considered to be unacceptable.
In contrast to a relative indifference to time delays in shore to ship communications, user messages between a central trucking dispatch depot and a fleet of operating trucks tend to be more time-sensitive. For example, interactive communication is required when a driver is in difficulty, e.g. by being lost or in misunderstanding with a customer. Urgent messages to alert drivers of additions or deletions in pick-up or delivery schedules while the trucks are already enroute are more the rule than the exception. Thus, if a Standard-C communication system operates between a xe2x80x9cland earth stationxe2x80x9d (xe2x80x9cLESxe2x80x9d) as a hub and a number of mobile terminal equipped, land-based vehicles, existing message transport delays of several minutes for typical command-response type transactions become undesirable.
Increasing the data transmission rate over the standard 600 bps data rate of the Standard-C system would result in a higher message data capacity per frame, thus allowing more mobile terminals to be serviced over the system. However a data rate increase over that of the prior art 600 bps would not alleviate the system""s inherent message transport delays. On the other hand, relatively shorter frame lengths could speed up handshake operations between transmitting and receiving terminals and would therefore reduce message transport delays. However, the use of frame lengths shorter than the standard 8.64 second frame length would, at any given transmission rate, tend to increase the ratio of network management data to user message data. Thus, shortening the frame length by simply increasing the data rate proportionally to the frame length reduction does not improve the aforementioned overhead ratio of network management information to user information. The frame structure comprising both network management data and user message data would become compressed in time but remain proportionally the same.
In addition to the undesirably long message transport delays described with respect to the Standard-C system, another drawback of the Standard-C system (and all other prior art FDMA/TDMA communication systems) is a well established dedication of frequency-division multiplexed channels as either signalling channels or message channel. For example, in communications originating at the mobile terminals, the first traffic component over the return channels, namely signalling, is assigned to the dedicated signalling return channels. Conversely, the second traffic component of user messages is assigned to the dedicated messaging return channels. It has become apparent that such dedicated use of available channels for communication from the mobile terminals to the hub results in an uneven utilization of the available channel capacities. As described above, signalling as used herein refers to the transport of protocol data packets for the purpose of network management, e.g. for call establishment and tear-down. Time division multiplexing within a signalling channel is accomplished by means of the slotted Aloha protocol, a contention-based channel access algorithm that does not guarantee packet delivery. In addition to the transport of network management protocol packets, it is also common to use the signalling channel for the transport of short data reports, provided they can tolerate non-guaranteed delivery, the use of signalling channel for such purpose being referred to as xe2x80x9cdatagramxe2x80x9d service. In contrast, user messages requiring guaranteed delivery are sent on separate frequency multiplexed time-division multiple-access (TDMA) channels which are contention free. It is these latter type of TDMA channels that are referred to as xe2x80x9cmessage channelsxe2x80x9d. Access to the message channels is controlled by the hub, and is orchestrated through instructions contained in protocol packets which are sent on the forward TDM channel and acknowledged or responded to over the return signalling channel. When typical user message comunication takes place over the Standard-C system, it is common to reach a traffic congestion limit in the TDM forward channel and the signalling channels before reaching a congestion limit in the message channels. Therefore, the message channels often remain underutilized.
It is therefore an object of the invention to lower message transport delays relative to message transport delays found in store-and-forward Standard-C communication systems.
Another object of the invention is to increase an effective information rate over the effective information rate of a Standard-C communication system, concurrently with lowering message transport delays with respect to message transport delays in such Standard-C communication system.
A further object of the invention is to optimize return channel capacity in leased channel groups.
Yet another object of the invention is to adapt the capacity of a fixed number of leased transmission channels to handle a range of ratios of signalling to messaging activities over return channels to more fully utilize the transmission channels before switching to new channel groups.
Accordingly, an improvement of time division multiple access (TDMA) communication systems intended for communicating information messages between a hub and a plurality of mobile terminals includes switches to transfer data at any of a number of data rates. Data are transmitted over the forward (TDM) channel in a frame format, the length of which is data rate dependent and shortened relative to frame lengths in Standard-C systems, to result in a reduction of message transport delays with respect to those of such Standard-C systems.
Specific examples of preferred, preestablished frame lengths, without limiting the scope of the invention, are a frame length of 1.0 second for data rates of 600 or 1200 bits per second (bps), a frame length of 0.5 second for a data rate of 2400 bps, and a frame length of 0.25 second for a data rate of 4800 bps.
Network management data are classified into time-critical and non-time-critical network management data. Non-time-critical network management data are consigned to a long bulletin board packet, referred to as xe2x80x9clong bulletin boardxe2x80x9d which is formatted into a first type of frame or login frame. Certain preselected, non-time-critical network management data are deleted from the long bulletin board packet to form a short bulletin board packet for a second type of frame, wherein the bulletin board data are referred to as a xe2x80x9cshort bulletin boardxe2x80x9d.
Time-critical network management data comprise, for example, return channel frequency assignments and slot timing for both signalling and messaging modes of return channel communication. The time-critical network management data are consigned to a return channel descriptor packet which is formatted into each frame, including the first type and, when used, the second type of frames as well as to a third type of frame or message frame. The message frame includes neither the long bulletin board nor the short bulletin board of non-time-critical network management data. The first type of frame or the second type of frame and a predetermined number of third type of frames or message frames are combined into first or second type of super frames of a predetermined length. The first and second type of super frames, when both are used, are transmitted alternately.
In a method of transmitting data over the forward channel in accordance herewith, time-critical network management data are transmitted periodically at a first rate, a frame transmission rate. Non-time-critical network management data are transmitted periodically at a second rate which is less than the frame transmission rate.
In a specific example, non-time-critical network management data are transmitted at the transmission rate of a superframe comprised of a series frames of one first type of frame and a predetermined number of third type of frames.
In a further specific example according to which certain parts of the non-time-critical network management data are selectively deleted from the long bulletin board to form the short bulletin board, the long bulletin board and the short bulletin board are transmitted periodically and alternately in even-numbered and odd-numbered superframes, respectively, such that each is respectively transmitted periodically at one-half of the super frame transmission rate.
Broadly viewed, network management data are categorized according to the degree of time criticality and are transmitted at a frame rate for time-critical network management data, at a superframe rate for non-time-critical network management data of a first order, and at a rate less than a superframe rate for non-time-critical network management data of higher order. The higher order denotes herein one or more categories of data of a lower transmission priority than the transmission priority assigned to the network management data of the first order.
More specifically, network management data are categorized into at least three different levels of time critical information and is formatted selectively into the three types of frames. The first type of frame contains least time-critical information. The second type of frame contains information with relatively more time-criticality than the least time-critical information. The third type of frame contains most time-critical information relative to the least and relatively more time-critical information.
Once a mobile terminal has logged in, message transport delays are determined by the transmission rate of the frames, resulting in greatly reduced message transport delays when compared to Standard-C systems. An advantage is a reduction, with respect to Standard-C systems, in a percentage overhead of network management data at forward channel data rates of 1200 bps or higher with concurrent reductions of message transfer delays over previously incurred message transfer delays of Standard-C systems.
Other features and advantages will become apparent from the detailed description set forth below.